package com.wenhao.newcode.pinduoduo.hard2020;

import java.util.Scanner;

/**
 * @author Wenhao Tong
 * @create 2021-07-25 17:04
 */
public class Main4 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int[] X = new int[n];
        int max = 0;
        for (int i = 0;i < n;i++) {
            X[i] = scanner.nextInt();
            max = max > X[i] ? max : X[i];
        }
        double[] p = new double[max + 1];
        for (int i = 1;i <= max;i++) {
            p[i] = 1.0;
        }
        for (int i = 1;i < max;i++) {
            for (int j = 0;j < n;j++) {
                if (i <= X[j]) {
                    p[i] *= (double)i / X[j];
                }
            }
        }
        double exp = 0;
        for (int i = 1;i <= max;i++) {
            exp += i * (p[i] - p[i - 1]);
        }
        System.out.println(exp);
    }
}
